A spiraling pool of cold, dense, arctic air called a "polar vortex" began its frigid sweep of the U.S. today. "It's just a dangerous cold," said National Weather Service meteorologist Butch Dye this morning. The arctic chill is expected to burden some regions of the country with the coldest weather they've seen in close to twenty years.

A "polar vortex" will affect more than half of the continental U.S. starting Sunday and into Monday and Tuesday, with wind chill warnings stretching from Montana to Alabama. The vortex is a counterclockwise-rotating pool of cold, dense air, and is behind the startling forecast: 25 below zero in Fargo, N.D., minus 31 in International Falls, Minn., and 15 below in Indianapolis and Chicago.

The bitterly cold temperatures already pushed into northern states Sunday morning. The National Weather Service reported a temperature of 9 below zero in Bismarck, N.D., and negative 21 at Duluth, Minn. At the height of the cold, wind chills may reach 50, 60 or even 70 below zero.

This cold front is no joke, people. For the first time in 17 years, Minnesota called off school Monday for the entire state. "A person not properly dressed could die easily in those conditions," warned National Weather Service meteorologist Scott Truett in St. Louis, describing the expected wind chill in Missouri tomorrow morning. The National Weather Service has described the wind chill as "life-threatening."